The present disclosure relates generally to circuit breakers and particularly to a trip unit for a circuit breaker.
Circuit breaker trip units and motor overload devices provide protection for cables, motors and other loads by measuring electrical current and simulating the heating of said cables, motors, and loads. One traditional approach uses bimetallic elements heated by current, another uses electronics to estimate heating as proportional to the current squared (I2R heating). In response to the opening of one of these devices, current is removed from the cables or loads to allow cooling. In electronic trip unit applications, many of these protective devices derive operating power from the load current and will de-energize and re-initialize following an open-close operation, allowing a user to reset the circuit breaker or motor overloads immediately. However, the cables, motors, and loads may have long thermal time constants and may not have had proper time to cool. This may cause equipment damage if heat arising from a first trip event remains in the motor or other load. Some thermal memory methods depend upon the storage of charge within a capacitor to simulate accumulated heat. However, in applications where the thermal time constants may be longer than capacitors can reliably hold an accurate charge, excessive heat may remain if a circuit breaker is turned back on too soon following a trip event. Alternative thermal memory methods may employ a thermal device that is heated by the current, such as a thermistor, for example. However, the load may have a larger thermal mass (and therefore take longer to cool) than some of the utilized thermal devices. While existing circuit breakers are considered suitable for their intended purpose, the art of circuit breakers may be advanced by providing a trip unit with enhanced thermal memory to improve protection for equipment with large thermal time constants.